I've got a Kenwood amp and Pioneer TS-W307D4 sub. For the amp i got the + from the battery way back to the trunk, but I can't find where in the trunk to mount the -. The problem is that everything is painted [paranoid] .
Any suggestions? 
I used a dremel with a grinding stone to get rid of the paint. I mounted my ground wire behind the driver side wheel well. There is a black plastic piece there. Just behind it. I drilled a hole through the trunk and bolted the ground down. It sits near the resonator from under the car. Have not had any issues with grounding there.
You can loosen a seat bolt and use that. I did this for my amp under the passenger seat. I loosened the console side rear bolt and used an O ring style wire end to under the seat bracket. You could do this under any seat. The cool thing is no worries will drilling a hole in the car.